The invention relates to a process for the preparation of substituted or unsubstituted diphenylamines by reaction of the cyclohexanones and anilines on which they are based in the presence of a rhodium-containing supported catalyst.
For the preparation of diphenylamine and derivatives thereof, German Offenlegungsschrift 2,331,878 has disclosed a process in which the starting materials are imines, such as N-cyclohexylidene-aniline and derivatives thereof, which are dehydrogenated in the gas phase in the presence of supported catalysts based on nickel, platinum, palladium or copper/chromium.
Furthermore, German Offenlegungsschrift 2,520,893 has disclosed the preparation of diphenylamine by catalytic dehydrogenation of compounds and/or mixtures of compounds composed entirely or in part of hydrogenated diphenylamine in the presence of a dehydrogenation catalyst containing nickel/chromium, aluminium, copper, manganese and alkali. Compounds of this type which are mentioned in the exemplary embodiments are binuclear aromatic imines.
Dehydrogenation of dicyclohexylamine to give diphenylamine over noble metal catalysts has also already been proposed (German Offenlegungsschrift 3,801,754).
The processes mentioned require starting materials which are relatively expensive to prepare, such as N-cyclohexylidene-aniline or dicyclohexylamine, and need to be improved if they are to be applied to a process to be carried out industrially.